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Subject: Re: [OMPI users] Bug MPI_Iscatter
From: George Bosilca (bosilca_at_[hidden])
Date: 2013-11-22 06:21:11


Pierre,

On Nov 22, 2013, at 02:39 , Pierre Jolivet <jolivet_at_[hidden]> wrote:

> George,
> I completely agree that the code I sent was a good example of what NOT to do with collective and non-blocking communications, so I’m sending a better one.
> 1. I’m setting MPI_DATATYPE_NULL only on non-root processes. The root has a real datatype. Why should both match when using MPI_IN_PLACE ?

Because it is a strong requirement of the MPI standard: the typemap of a send should be matched by its corresponding receive. Otherwise, it is legal to raise an exception of type MPI_ERR_TYPE.

> 2-3-4. Yes, all these points are valid, this is of course just a minimalist example.
>
> My question is, if you are indeed saying that it is not a OpenMPI bug, what is the rationale for changing the behavior between MPI_Scatter and MPI_Iscatter when it comes down to the send type on non-root processes.

Different algorithms implemented by different people. Some of them are more robust, while others less. In this case Scatter translate the count = 0 to a message length = 0, while the Iscatter always look for the extent of the datatype.

> I don’t see any remark on that matter on the MPI 3.0 documentation.

Indeed, and there is at least one example where MPI_DATATYPE_NULL is explicitly used for calls where the datatype does not matter (4.23 as an example). Horrible!!!

  George.

>
> Thanks.
>
> #include <mpi.h>
>
> int main(int argc, char** argv) {
> int taskid, ntasks;
> MPI_Init(&argc, &argv);
> MPI_Request rq;
>
> MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
> MPI_Comm_size(MPI_COMM_WORLD,&ntasks);
> double* r;
> int l = 0;
> // This will run fine. MPI_DOUBLE
> if(taskid > 0)
> MPI_Iscatter(NULL, 0, MPI_DOUBLE, r, l, MPI_DOUBLE, 0, MPI_COMM_WORLD, &rq);
> else
> MPI_Iscatter(r, l, MPI_DOUBLE, MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, 0, MPI_COMM_WORLD, &rq);
> MPI_Wait(&rq, MPI_STATUS_IGNORE);
> // This will run fine. MPI_DATATYPE_NULL
> if(taskid > 0)
> MPI_Scatter(NULL, 0, MPI_DATATYPE_NULL, r, l, MPI_DOUBLE, 0, MPI_COMM_WORLD);
> else
> MPI_Scatter(r, l, MPI_DOUBLE, MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, 0, MPI_COMM_WORLD);
> // This will not run fine. MPI_DATATYPE_NULL
> if(taskid > 0)
> MPI_Iscatter(NULL, 0, MPI_DATATYPE_NULL, r, l, MPI_DOUBLE, 0, MPI_COMM_WORLD, &rq);
> else
> MPI_Iscatter(r, l, MPI_DOUBLE, MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, 0, MPI_COMM_WORLD, &rq);
> MPI_Wait(&rq, MPI_STATUS_IGNORE);
> MPI_Finalize();
> }
>
> On Nov 21, 2013, at 4:34 PM, George Bosilca <bosilca_at_[hidden]> wrote:
>
>> Pierre,
>> There are several issues with the code you provided.
>>
>> 1. You can’t use an MPI_DATATYPE_NULL as the send datatype, not even when count is zero. At least the root must provide a real datatype. In fact the type signature of the send message (datatype and count) should match the type signature of the receiving datatype.
>>
>> 2. I know your count is zero, and no data will be transmitted but your code is difficult to read and understand.
>>
>> 3. MPI_Iscatter is a collective communication. As such all processes in the associated communicator (MPI_COMM_WORLD in your case) must participate to the collective. Thus, calling MPI_Iscatter only where tasked > 0 is incorrect (you explicitly excluded 0).
>>
>> 4. From the MPI standard perspective your example is not correct, as you are not allowed to call MPI_Finalize while there are messages pending. Now, Open MPI tolerate this, but it is clearly not standard behavior.
>>
>> #include <mpi.h>
>>
>> int main(int argc, char** argv)
>> {
>> int taskid, ntasks;
>> MPI_Init(&argc, &argv);
>> MPI_Request rq;
>> MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
>> MPI_Comm_size(MPI_COMM_WORLD,&ntasks);
>> double r;
>> int l = 0;
>>
>> MPI_Iscatter(NULL, 0, MPI_DOUBLE, &r, l, MPI_DOUBLE, 0, MPI_COMM_WORLD, &rq);
>> MPI_Wait(&rq, MPI_STATUS_IGNORE);
>>
>> MPI_Finalize();
>> }
>>
>> George.
>>
>>
>> On Nov 21, 2013, at 23:19 , Pierre Jolivet <jolivet_at_[hidden]> wrote:
>>
>>> Hello,
>>> The following code doesn’t execute properly :
>>> #include <mpi.h>
>>>
>>> int main(int argc, char** argv) {
>>> int taskid, ntasks;
>>> MPI_Init(&argc, &argv);
>>> MPI_Request rq;
>>>
>>> MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
>>> MPI_Comm_size(MPI_COMM_WORLD,&ntasks);
>>> double* r;
>>> int l = 0;
>>> if(taskid > 0)
>>> MPI_Iscatter(NULL, 0, MPI_DATATYPE_NULL, r, l, MPI_DOUBLE, 0, MPI_COMM_WORLD, &rq);
>>> MPI_Finalize();
>>> }
>>>
>>> Outcome:
>>> *** An error occurred in MPI_Type_extent
>>> *** MPI_ERR_TYPE: invalid datatype
>>>
>>> Hotfix: change MPI_DATATYPE_NULL to something else.
>>>
>>> Thanks for a quick fix.
>>> _______________________________________________
>>> users mailing list
>>> users_at_[hidden]
>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>
>> _______________________________________________
>> users mailing list
>> users_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
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